This invention relates to a manufacturing control method for manufacturing discrete workpieces. Specifically the invention relates to a method for controlling a manufacturing apparatus and process whereby one or more dimensions of manufactured discrete workpieces may be closely controlled and wherein the apparatus is continuously adjusted to produce workpieces centered about a predetermined fixed mean value. Furthermore, the invention relates to a method for performing a capability study of the manufacturing process and apparatus and for statistical analysis and print-out of the data relating to the controlled dimension of manufactured workpieces.
In most manufacturing processes constant adjustment of the process is necessary due to variations in the material used, changes in ambient temperature, tooling wear and the like. Such variations and changes give rise to variations in the dimensions of the workpieces being produced. Thus, when a dimension is critical, the manufacturing process must be controlled to keep the dimension within tolerance limits.
Conventional process control in manufacturing discrete workpieces is generally accomplished by means of a guage which measures and sorts workpieces as they are produced. Generally upper and lower tolerance limits are established for the controlled dimension and workpieces are individually measured and sorted in accordance with the established tolerance limits as the workpieces are produced. Thus, if the critical dimension is within the established limits, the workpiece is passed as a good workpiece, whereas if the workpiece is not within the established limits the workpiece is rejected as scrap. If a large number of rejected workpieces is encountered, the process may be manually readjusted to achieve a reduction in the scrap workpieces and to increase the probability of manufacturing workpieces which are within the established tolerance limits. Thus, if the established limits are relatively close together, a larger amount of scrap is generally produced. This, of course, reduces the yield of the machine and results in added expense, thereby increasing the cost of the acceptable workpieces. Thus, from a practical standpoint, conventional process control is incompatible with tight tolerance limits.
An additional problem with the conventional type of process control is that the operator will only become aware of a problem in the manufacturing process after a large number of unacceptable workpieces has been produced. Normally the only feed back the machine operator receives is by means of lights on the gauge console to indicate that workpieces are being produced which are not within the set limits and therefore are unacceptable. Thus the system is slow to respond to variations in the process to adjust the manufacturing equipment for changes in material, ambient temperatures, tool wear and the like which normally occur.
A yet further problem with conventional manufacturing systems is the inability to provide data regarding the statistical distribution of the manufactured individual workpieces. While it is generally assumed that acceptable workpieces will have a normal distribution, it is entirely possible that the distribution of acceptable workpieces is skewed, whereby a larger number of workpieces are located close to either the upper or the lower established tolerance limit. While such workpieces are acceptable because they fall within the preset tolerance limits, it has been found that in an assembly having a large number of parts, the stack-up of tolerances may cause failure of the assembly. It is therefore desired to manufacture workpieces within tight tolerances to reduce the stack up of tolerances of an assembly. However, as pointed out above, conventional processes are unable to provide tight tolerances without excessive cost.
It is therefore desired to provide a manufacturing process wherein the workpieces are centered about a fixed mean. It is furthermore, desired to provide a process which is automatically adjusted, whereby the expected dimension of each current workpiece is closer to the fixed mean than the last produced workpiece.
A number of prior art types of statistical process controls (SPC's) have been provided for controlling continuous manufacturing processes. One example of such an SPC is given in U.S. Pat. No. 3,515,960. In this process the controller maintains the controlled material property as near as practicable to a set point value and operates the process closer to or further from the proscribed limit depending upon the controllability of the process as measured by the statistical variance of the process, while continuously monitoring the process to provide assurance that the limit will not be violated in the event that the controllability of the process deteriorates either slowly or rapidly. Thus this process does not control the controlled property of the material around a centered mean and therefore is not a center seeking system. Rather this process is a boundary avoidance control system with a variable mean and wherein the mean of the distribution is varied toward or away from the boundary or limit depending upon the controllability of the process. This process is therefore intended only to avoid producing material which is outside of the lower limit and therefore only attempts to avoid the production of scrap. The process is not intended to produce material centered around a fixed mean set limit. Furthermore, the process is intended for application to a continuous process rather than to the manufacture of discrete workpieces.
U.S. Pat. No. 3,946,212, discloses a manufacturing process for producing discrete workpieces. In this process individual workpieces are measured and a statistical analysis is performed on the manufactured workpieces to determine if the process is producing workpieces within the established tolerance limits. If it is found that the process produces unacceptable parts, the operator is alerted and the process is manually adjusted. Thus this system is also not a center seeking system but is instead a boundary avoidance system. Furthermore, the system includes no automatic control for continuously automatically adjusting the machine to achieve a centered distribution. Lastly, no sorting is performed on the workpieces which are produced by the process.
U.S. Pat. No. 3,147,370 discloses a system wherein production information is automatically collected, collated and operated on so that the data may be printed out for comparison with standards without laborious manual computation. However, the system shown does not include any means for controlling the process or for automatically adjusting the manufacturing process to produce workpieces which are centered around a fixed mean.